1. Field of the Invention
Field of the present invention relates generally to NOx reduction of diesel engines having an emission control device that stores NOx during certain conditions and releases/reduces stored NOx during other operating conditions. More particularly, the field of the present invention relates to system configurations to reduce component part costs.
2. Background of the Invention
Lean burn engines, such as diesel and gasoline engines, can provide increased fuel economy and power density. One approach to reduce NOx emissions uses a catalytic converter in the engine exhaust. One type of catalytic converter stores NOx when the engine is running lean, and releases/reduces the stored NOx when a rich exhaust gas enters the catalytic converter. When using a diesel engine, one approach to provide rich exhaust gases to the catalytic converter uses a fuel injector positioned in the engine exhaust.
However, to minimize the amount of excess fuel added to create the rich exhaust gas (due to the large amount of excess oxygen when running lean), a split exhaust gas pipe, each containing a fuel injector and a catalytic converter, is utilized. Further, a valve assembly is also used that directs the flow to either branch, and allows reduction of the flow to the branch that is receiving the rich exhaust gas. This requires the downstream injector to overcome only a smaller amount of excess oxygen. Such a system is described in SAE Paper No. 2001-01-3619.
The present inventors, however, have recognized a disadvantage with such an approach. In particular, such a configuration contains two exhaust valves (one for each branch) and two fuel injectors in the exhaust pipes (again, one for each branch). This additional hardware significantly increases system costs, as well as manufacturing complexity and wiring complexity.
The trucking industry is extremely cognizant of increased capital and operating costs of emissions related technology. As described in a recent newspaper article (see xe2x80x9cFleets Wary of New Dieselsxe2x80x9d, May 31, 2002, The Detroit News, pg. 3B), additional cost of new technology is a major impediment to its implementation.
The above disadvantages are overcome by a system comprising: an engine having an exhaust system through which exhaust gasses flow; a first and second emission control device in said exhaust system of said engine; an injector in said exhaust system that injects a reductant; and an exhaust valve in said exhaust system upstream of said first and second emission control device, said valve having at least a first and second position, said first position creating a first path for said reductant from said injector to reach said first emission control device, and a second position creating a second path for said reductant from said injector to reach said second emission control device.
In this way, it is possible to minimize reductant needed to purge a NOx catalyst without the added cost and complexity of requiring multiple injectors. I.e., by this judicious plumbing, the present invention can achieve the same ability of directing lean exhaust flow of a large magnitude, and rich exhaust flow of a small magnitude with reduced system costs and complexity. In other words, the configuration according to the present invention can achieve the required functionality while eliminating a fuel injector and an exhaust control valve. Further complexity reduction and cost saving is achieved since there is similarly a reduction in wiring and plant manufacturing operations and complexity.
In another aspect of the present invention, the above disadvantages are overcome by a vehicle system comprising: a diesel fueled engine having an exhaust manifold through which exhaust gasses flow; an exhaust valve having at least first, second, third, and fourth unions, said valve directing exhaust gas from said first union to both said second and fourth unions and a second medium from said third union to said fourth union when in a first position, and directing said exhaust gas from said first union to both said second and fourth unions and said second medium from said third union to said second union when in a second position; an injector coupled to said third union that injects said second medium; a first emission control device coupled to said second union; and a second emission control device coupled to said fourth union.
In one particular example, a reductant, such as diesel fuel, or urea, can be used as the second medium.
Note that there are various types of valves that can be used to direct exhaust gas flow and reductant as claimed described above. For example, vacuum actuated or electromechanically actuated poppet valves can be used. Similarly, direct electronic solenoid valves can be used. Further still, hydraulically actuated valves, or any combination of the above, could be used. Note also that various types of emission control devices can be used. For example, catalyst comprising platinum on a carrier can be used. Further still, barium or other such elements can be added. Alternatively, zeolite type NOx catalysts could be used. Note also that there can be additional devices in between the fourth union and the second emission control device. Similarly, there can be additional devices in between the second union and the first emission control device. Also, there can be devices between the first union and the exhaust manifold of the engine (e.g., upstream catalyst, muffler, or particulate storage device). Finally, there can be addition devices, such as an air assist device between the third union and the reductant injector. Further still, other devices can be coupled to any of the at least four unions.
In another aspect of the present invention, disadvantages with prior approaches are overcome by a method for controlling an engine, the engine having an exhaust through which exhaust gasses flow, said exhaust having at least a first an second catalyst and at least a one reductant injector, the method comprising: providing a first portion of the exhaust gas flow to said first catalyst and a second portion of the exhaust gas flow to said second catalyst; operating in a first mode where said first portion is greater than said second portion; during at least a first interval while in said first mode, injecting reductant from said at least one reductant injector into said second portion of exhaust gas flow; operating in a second mode where the second portion is greater than said first portion; and during at least a second interval, while in said second mode, injecting reductant from said at least one reductant injector into said first portion of exhaust gasses.
In this way, it is possible to minimize the amount of necessary reductant used from the first reductant injector. Further, preferably, only a single reductant injector is needed.
Note that the first and second intervals can be of different lengths. Further note that the intervals can vary depending on operating conditions. For example, a time interval can be used. Alternatively, the interval can be ended based on estimates of NOx stored and reduced in the catalysts. Further, the interval can be set based on outputs of sensors coupled downstream of the catalysts.